Digital optical comparator

ABSTRACT

A digital optical comparator has a holder for a part under study. A light source illuminates the part and casts an image of the part onto a camera, which is provided with a lens. The image captured by the camera is displayed on a screen, and a drawing of the part is overlaid on the image of the part. Thus, defects in manufacturing can be easily and readily identified. In addition, a determination of whether the part is manufactured within tolerances can also be visually determined.

FIELD OF THE INVENTION

The present invention relates to a digital optical comparator.

DESCRIPTION OF THE PRIOR ART

An optical comparator (often simply called a comparator, or a profileprojector or a contour projector) is a device that applies theprinciples of optics to the inspection of manufactured parts. In acomparator, the magnified part is projected on a screen, and thedimensions and geometry of the part are measured against prescribedlimits.

Prior art comparators rely heavily on the skill of the operator toproperly align the part and the reference template for comparisonpurposes. Additionally, prior art comparators do not provide astraightforward means for orienting the part and the reference templateonce the part is installed in the comparator.

Some of the prior art comparators consist of a light source, a supportfor the part, and optics for capturing the image of the part anddisplaying the same on a display. The display usually displays a zoomedin image of the part. In these comparators, a transparency of thedrawing of the part, or a blueprint, or some other printed/drawn imageis actually taped to the screen of the comparator in the properorientation in order to compare the part with the drawing. It will beappreciated that this is quite intensive on the part of the operator.Not only must the drawing be to the correct scale, as must be the imageof the part on the screen, it is delicate to properly overly thepaper/printed drawing over the screen in order to compare the part tothe drawing.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a digital opticalcomparator which overcomes the deficiencies of the prior art.

Thus, in accordance with one aspect of the invention, there is provideda digital comparator comprising:

a holder for holding a part;

a light source for illuminating the part;

a camera for capturing an image of the illuminated part and a lensplaced in front of said camera; and

a data processing system for receiving the image, for obtaining a CADdrawing of said part, for displaying said image and said drawing on acomputer screen, where said drawing is digitally overlaid on said image,said data processing system further including means for aligning saiddrawing with said image.

In accordance with another aspect of the invention, there is provided adigital optical comparator comprising a method for permitting comparisonbetween a part under study and a reference drawing, said part havingbeen manufactured to specifications contained in said drawing, saidmethod comprising the steps of:

(a) placing said part on a support;

(b) illuminating said part;

(c) capturing an image of said part with a camera, said camera beingprovided with a lens; and

(d) overlaying said drawing and said image of said part on a display.

DESCRIPTION OF THE FIGURES

The present invention will be better understood after having read adescription of a preferred embodiment thereof made in reference to thefollowing drawings in which:

FIG. 1 is a schematic representation of a digital optical comparatorsystem according to a preferred embodiment of the invention;

FIG. 2 is a schematic representation of a digital optical comparatorsystem according to another preferred embodiment of the invention, whereboth the light source and the camera are on the same side of the part;

FIG. 3 is a schematic representation of a holder for the part;

FIG. 4 is a view of the part displayed on multiple displays, with thedrawing superimposed thereon, but in a non-oriented position;

FIG. 5 is a view of the part displayed on multiple displays, with thedrawing superimposed and aligned with the image of the part; and

FIG. 6 is a schematic representation of a data processing system for thedigital optical comparator system.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION GeneralDescription

The present invention concerns a digital optical comparator system,which will, for ease of reference, be hereinafter referred to as “DOC”.The DOC of the present invention is a computerized optical instrumentthat allows the user to compare a digital live video image of a partwith its CAD drawing, without any restriction regarding the part's size,and in real-time (i.e. at the camera's full frame rate).

The DOC captures a very high-resolution, digital live video image of apart and in real-time carries out image correction operations to removeany distortion introduced by the system's lens and correct for anydefective pixels in the image.

Following these image correction operations, the DOC produces ageometrically correct, high-quality image of the part (i.e. un-distortedand without point defects), for both big and small parts.

Still in real-time, the DOC:

-   -   carries out any required image processing operations on the        “corrected” image (for example: image enhancement, uneven        illumination correction, noise removal, color correction, etc. .        . . )    -   superimposes the part's CAD drawing onto the “corrected” and        “enhanced” image.

The DOC allows the user to move the CAD drawing on screen (X-Ytranslation & rotation) using either an external device (such as ajoystick with X-Y-Theta controls, for example) or equivalent on-screencontrols for X-Y translation & rotation. This allows the user to quicklyand easily line up the part and its drawing, without having to move thepart.

Using pattern matching software tools, the DOC can also carry out thisalignment operation automatically.

Finally, the DOC displays, at the camera's full frame rate, theresulting “corrected” and “enhanced” high-resolution video image, withthe superimposed properly scaled and aligned CAD information, in fullresolution on at least one computer monitor.

With the DOC, if the part matches its CAD drawing exactly, then thepart's image and the CAD drawing line up perfectly. If there are anydiscrepancies between the part and its CAD drawing these discrepancieswill be readily visible on-screen.

The DOC also allows the operator to measure various dimensionson-screen, including the size of any such discrepancies.

Using edge-detection based measurement tools, the DOC can alsoautomatically verify that the part is within tolerance at varioususer-specified control positions.

It will be appreciated that the DOC can also include a stage, eithermanual or motorized, to move the part in front of the camera. When thestage is encoded the DOC is able to carry out measurements beyond theoptical field of view (by relating the stage displacements and theon-screen position of features between measurement points).

It will further be appreciated that the DOC can call-up a part's drawingsimply by either scanning in a barcode (which might correspond to a workorder, for example) or by manually typing in the barcode information (inour example: typing-in the work order number).

Note also that the DOC has the ability to recall a previously-displayedCAD drawing (as well as the position and orientation at which it wasdisplayed) by clicking on a single on-screen button.

The DOC consists of:

-   -   A lens 11    -   A digital camera 13    -   A computer, or other appropriate data processing system, 15        running the appropriate hardware and software to interface with        the camera    -   One or more monitors 17. A preferred implementation provides for        multiple monitors which allows the high-resolution image        produced by the camera to be displayed fully (i.e. in 1:1        resolution on the bank of monitors, so that every pixel coming        from the camera corresponds to a pixel on the bank of monitors,        and allows for room for other windows).    -   Illumination means 19 (either transmitted and/or reflected, for        back- and/or front-illumination, respectively). A preferred        implementation of the DOC provides that the illumination is        programmable and computer-controlled, which allows for        repeatable illumination conditions. The reader will appreciated        that programmable, computer-controlled illumination requires        that appropriate interface and control hardware be present.    -   Means 21 for mounting the camera, lens and illumination, such as        brackets, hardware and structural elements. The reader will        appreciate that the camera, lens and illumination means can be        mounted either vertically, horizontally or at any other        arbitrary orientation.    -   Means 23 for mounting the part 20 between the camera and the        lens.    -   Adapted software running on the computer.

Implementation and Operation Image Capture

The DOC's illumination, lens and camera produce a stream of very highresolution digital images (also known as live video) of a part. This isa difference from some prior art systems in that a “snapshot” or stillimage of the part is captured. In the present invention, streaming datais obtained, so that a more dynamic comparison of the part can be made.

A preferred implementation uses a telecentric lens and includesprogrammable, computer-controlled illumination (for repeatableillumination conditions).

Image Correction

The DOC carries out real-time image correction operations of this imagestream.

In a preferred implementation, automatic image correction 31 is carriedout to obtain a geometrically correct image of the part (i.e. an imagein which all of the pixels correspond to the same real-worlddimensions). The DOC's automated distortion correction subsystem isdescribed in detail below.

This scheme corrects errors introduced by the DOC's lens. Without anysuch correction, the image would not be geometrically true to the part(i.e. certain portions would appear larger than they should and otherssmaller). This is especially true for larger field of views where alloptical lenses distort the image that they produce, to a greater orlesser degree.

The DOC also corrects for defects in the image introduced by the camera.Our preferred implementation corrects the image for defective pixels,also referred to as dead pixels. The DOC's automated defective pixelcorrection scheme is described further.

A preferred implementation of the invention uses a camera with a veryhigh resolution sensor and these sensors are precisely the ones in whichdefective pixels are more common. The DOC automatically identifiesdefective pixels and corrects the image to remove their effect.

The image correction allows the DOC to produce a geometrically correct,high-quality image of the part (i.e. un-distorted and without pointdefects), for both big and small parts.

Image Enhancement

Following the image correction schemes, the DOC also carries outreal-time image enhancement 33 on each image in the live video stream.

In a preferred implementation of the DOC, some of the image enhancementoperations that are available include, without limitation: noiseremoval, uneven illumination correction, edge sharpening and colorcorrection.

CAD File Comparison

The DOC can be used to read in a CAD file containing the mathematicaldefinition of the part. The preferred implementation includes a CADpackage 37 which reads in DXF format CAD files from a database 45 orother repository, but other CAD file formats (such as DWG) are alsowithin the scope of the present invention.

The DOC can automatically scale the contents of the CAD file to match upwith the system's optical magnification.

The appropriately scaled CAD data can then be superimposed onto thestream of (corrected and enhanced) live video images

Manual Alignment of the CAD Data and the Part

The DOC allows the user to manually align the CAD drawing with the imageof the part.

The DOC allows the user to move the CAD drawing on screen (X-Ytranslation and rotation) using either an external device (such as ajoystick 41 with X-Y-Theta controls, for example) or equivalenton-screen controls for X-Y translation and rotation, for example with akeyboard and mouse 43, or even by using trackballs or touch-screens.This allows the user to quickly and easily line up the part and itsdrawing, without having to move the part.

Automated Alignment of the CAD Data and the Part

Using pattern matching software tools 39, the DOC can also carry outthis alignment operation automatically.

A preferred implementation uses the following approach: the first timethat a CAD file is read into the system (we will say that the system isthen in “teach” mode), the user aligns the part's drawing manually withthe displayed image of the part. The system then remembers this positionand orientation. We will call the position and orientation of the partin “teach” mode the nominal position and orientation.

Then, the next time that the drawing is read-in to the system, a patternmatching tool can be used to find the part's position and orientation.The rotation and translation difference between this position and thenominal position can then be applied to the CAD data. This way, the CADdrawing always “follows the part”.

A further refinement is also possible. The position and orientationfound using the approach described above is used as a starting positionfor a “fine search”. This “fine” search uses a certain number of“control points” spread reasonably uniformly throughout the CAD drawing.A software edge detection tool then finds the edge point closest to eachof these “control points”. Keeping constant the distance between all ofthe various edge points (i.e. “rigid body motion”), the DOC attempts tocarry out fine translation and rotation adjustments to the drawing'sposition and orientation so as to minimize the square root of the sum ofthe squared distance between each of the control point and its nearestedge (i.e. “least squares” approach).

Image Display

The resulting stream of images (with the superimposed CAD data, properlyscaled and aligned) can be displayed on-screen in real time by thecomputer, at the camera's full frame rate.

In the preferred implementation, the image stream is displayed ontomultiple monitors, in full 1:1 resolution (i.e. every pixel in the imageis displayed using one pixel in the “monitor array”).

With the DOC, if the part matches its CAD drawing exactly then thepart's image and the CAD drawing line up perfectly on-screen. On theother hand, if there are any discrepancies between the part and its CADdrawing then these non-conformities will be readily visible on-screen.

Measurements

The DOC also allows the operator to measure various dimensions, bysimply selecting measurement points on-screen with the mouse, includingthe size of any such discrepancies. The coordinates are provided by andinterpreted with a measurement package 51.

The DOC allows the user to carry out simple point-to-point measurements,as well as many other types of measurements: angle, diameter, radius,etc. . . . Measurements to and from elaborate geometric constructs, suchas from the center of a circle, the point of intersection of two lines,etc, are also possible.

Using edge-detection based measurement tools, the DOC can alsoautomatically verify that the part is within tolerance at varioususer-specified control positions.

Stage Interface

The DOC can also include a stage 23, either manual or motorized, to movethe part in front of the camera. When the stage is encoded the DOC isable to carry out measurements beyond the optical field of view, byrelating the stage displacements and the on-screen position of featuresbetween measurement points.

Other Capabilities

The DOC can call-up a part's drawing simply by either scanning in abarcode (which might correspond to a work order, for example) or bymanually typing in the barcode information (in our example: typing-inthe work order number).

The DOC also has the ability to recall a previously-displayed CADdrawing (as well as the position and orientation at which it wasdisplayed) by clicking on a single on-screen button.

Optical Distortion Correction

Optical aberrations are due to the departures from the idealizedconditions of Gaussian optics.

There are two main types of optical aberrations: chromatic andmonochromatic aberrations. Chromatic aberrations are related to thedifferent wavelengths in light, whereas monochromatic aberrations occureven if the light is highly monochromatic.

Distortion is one of the five primary monochromatic aberrations thatstem from the differences between first-order and third-order paraxialtheory. The other four are:

-   -   spherical aberration    -   comatic aberration (or coma)    -   astigmatism and    -   field curvature

Spherical aberration, coma and astigmatism deteriorate the image andmake it unclear, whereas field curvature and distortion deform theimage.

Optical distortion occurs when the transverse magnification of a lens isnot constant, and varies as a function of the off-axis image distance.If the magnification increases with the off-axis distance, then eachpoint is displaced radially outward from the center of the image, withthe most distant points moving the greatest amount. This is knows aspositive or pincushion distortion. Similarly, negative or barreldistortion corresponds to the situation where the magnificationdecreases with the axial distance and in effect each point on the imagemoves radially inward toward the center.

A Mathematical Model of Optical Distortion

In the case of a rotationally symmetric optical system, radialdistortion can be approximated by the following equation:

x′=x(1+k|x| ²)  (1)

Where:

x is the undistorted position (measured radially)

x′ is the distorted position (measured radially)

k is a constant that depends on the properties of the lens.

k is the coefficient of distortion. Clearly, k=0 corresponds to asituation where there is no distortion and positive and negative valuesfor k results in positive and negative distortion, respectively.

Determining the Distortion Coefficient Single-Sided Method

[One method to determine a lens' distortion coefficient involves imaginga standard with three aligned fiducials. The first fiducial is placedexactly in the center of the camera's field-of-view. The spacing betweenthe fiducials, d, is constant. These conditions are shown below.

Note that standard meeting these requirements are quite common. A stagemicrometer, for example, could be used.

Under these conditions, equation (1) can be written as follows:

X′ _(d) =d(1+k|d| ²)

X′ _(2d)=2d(1+k|2d| ²)

In these equations, the subscripts d and 2 d refer to the fiducials atpositions d and 2 d away from the center of the optical centerline,respectively.

Note that in equations (2a) and (2b) the terms x_(d)′ and x_(2d)′ areknown (i.e. they can be measured by the imaging system), whereas d and kare unknown. Of course, d is arbitrary and of no interest and ourobjective here then is to obtain k.

Also, note that throughout this entire document, positions and distancesare always expressed in pixels.

Equations (2a) and (2b) can be written as follows:

X′ _(d) =d+kd ³

X′ _(2d)=2d+8kd ³

And isolating k in both of these, we obtain:

$k = \frac{x_{d}^{'} - d}{d^{3}}$$k = \frac{x_{2d}^{'} - {2d}}{8d^{3}}$

These two equations can then be combined to produce

$\frac{x_{d}^{'} - d}{d^{3}} = \frac{x_{2d}^{'} - {2d}}{8d^{3}}$

Which can be solved for d as follows:

$d = {{\frac{4}{3}x_{d}^{'}} - {\frac{1}{6}x_{2d}^{'}}}$

d can then be substituted in order to obtain k.

Double-Sided Method

A second method involves using five evenly spaced fiducials, as shown inthe following image.

With this approach, we align the middle fiducial as much as possiblewith the center of the image. We then measure the distance between thetwo “inner” fiducials, which we will refer to as x′_(inner), and thedistance between the two “outer” fiducials, which we will callx′_(outer).

Under these conditions, equation (1) can be written as follows:

X′ _(inner)=2d(1+k|d| ²)

X′ _(outer)=4d(1+k|2d| ²)

Equations and can then be written as follows:

X′ _(inner)=2d+2kd ³

X′ _(outer)=4d+16kd ³

And isolating k in both of these, we obtain:

$k = \frac{x_{inner}^{'} - {2d}}{2d^{3}}$$k = \frac{x_{outer}^{'} - {4d}}{16d^{3}}$

which can then be combined to produce

$\frac{x_{inner}^{'} - {2d}}{2d^{3}} = \frac{x_{outer}^{'} - {4d}}{16d^{3}}$

Which can be solved for d as follows:

$d = {{\frac{2}{3}x_{inner}^{'}} - {\frac{1}{12}x_{outer}^{'}}}$

d can then be substituted to obtain k.

It can be shown that the double-sided method is twice as accurate as thesingle-sided method. The double-sided method should thus always beretained whenever practical.

The reader will appreciate that with both the single-sided anddouble-sided methods, all measurements should always be carried out fromfiducial to fiducial, never from the system's optical centerline. Thisvirtually eliminates any error due to small misalignments between theartifact and the optical centerline.

Practical Implementation

Implementing automatic distortion correction requires a calibrationprocedure as well as a fast implementation of equation (1).

Calibration Procedure

A semi-automated distortion calibration procedure is contemplated by thepresent invention, based on the double-sided method described above.

During the calibration operation, a crosshair will be displayed, toallow for alignment of the artifact under the optics (i.e. to align theartifact with the center of the field-of-view and to “square up” theartifact with the field-of-view.

The end result of the calibration procedure is the coefficient ofdistortion k.

Carrying Out the Distortion Correction “on the Fly”

With the k coefficient of distortion known, we can carry out apre-processing operation to determine the “source” coordinates for everypixel in the image.

Specifically, this means that we need to solve (1) (which we re-writebelow for convenience) for x′, with x and k known.

x′=x(1+k|x| ²)

In the general case, of course, x′ will not be an exact integer value.This means that it falls somewhere between pixels, rather than exactlyon one.

We must then carry out interpolation (of a chosen order) to obtain a“correspondence matrix” (i.e. information at pixel (i,j) should be takenfrom this location, according to that equation) that can be implementedpractically.

Nearest-Neighbor Interpolation

For example, we can carry out nearest-neighbor interpolation (i.e.0^(th) order interpolation), in which case the correspondence matrix isthe simplest (i.e. information at pixel (i,j) should be taken directlyfrom pixel (k, l)).

Nearest-neighbor interpolation is a very fast operation; however itproduces a resulting image with “blocky” artifacts. It can be shown thatnearest-neighbor interpolation can result in a spatial offset error upto ½^(1/2) pixel units.

Bilinear Interpolation

Bilinear interpolation (i.e. 1^(st) order interpolation) usuallyproduces better results, for an extra computational cost.

The general equation for bilinear interpolation is

v(x,y)=ax+by+cxy+d

Where:

v(x,y) is the interpolated value at position (x,y)

a, b, c and d are coefficients

Note that in this form of the bilinear equation:

0<=x<=1

0<=y<=1

Using the known information v(0,0), v(0,1), v(1,0) and v(1,1), the fourcoefficients of can be expressed as follows:

a=v(1,0)−v(0,0)

b=v(0,1)−v(0,0)

c=v(1,1)−v(1,0)+v(0,0)−v(0,1)

d=v(0,0)

In the case of bilinear interpolation, the correspondence matrix can beimplemented by keeping, for every pixel, the four “corner” coordinates(i.e. coordinates (0,0), (0,1), (1,0) and (1,1) in equation (13)) aswell as the x and y values used to solve equation. This information canthen be used to solve the equations in real time.

Note that for a color image, all three color channels must beinterpolated in this way. This is true for both nearest-neighbor andbilinear interpolation.

Finally, note that other higher-order interpolation schemes are alsoavailable and can, in many cases, produce better results. However, thecomputational cost is generally much higher. Bilinear interpolation isgenerally regarded as a very good compromise.

Defective Pixels

In the context of the present invention, defective pixels are pixels ona camera's CCD sensor that do not perform as they should. Defectivepixels can affect both CCD and CMOS sensors, although in Applicant'sexperience they are much more prevalent in CMOS sensors, especiallythose with a very high resolution.

Defective pixels can also occur on LCD monitors. Whereas in camerasensors defective pixels fail to sense light levels correctly, in thecase of LCD monitors defective pixels fail to produce the correct lightlevels.

The ISO 9002 13406-2 standard, specifically for LCD screens,distinguishes between three different types of defective pixel:

1. Hot pixels (always on)

2. Dead pixels (always off)

3. Stuck pixels (one or two sub-pixels are always on or always off)

Still according to the ISO 9002 13406-2 standard:

Hot pixels are usually best seen against a dark background. Apermanently lit white pixel, called a glowing pixel, is a special caseof a hot pixel.

A dead pixel is a defective pixel that remains unlit. Dead pixels areusually best seen against a white background.

A stuck pixel will usually be most visible against a black background,where it will appear red, green, blue, cyan, magenta, or yellow,although stuck red, green, or blue pixels are most common. Each pixel onan LCD monitor is composed of three sub pixels (one red, one green, andone blue) which produce the visible color of the pixel by their relativebrightness. A stuck pixel in an LCM monitor results from a manufacturingdefect, which leaves one or more of these sub-pixels permanently turnedon or off.

Defective pixels in LCD screens can sometimes be improved bymechanically manipulating the area around a defective pixel by pressingor tapping. This can help to evenly distribute the oil inside thescreen, but it can also damage it.

Stuck pixels are not guaranteed to be correctable, and can remain faultyfor the life of the monitor, however might be fixed by flashing numerouscolors with a very rapid intensity.

Stuck pixels are often incorrectly referred to as dead pixels, whichhave a similar appearance. In a dead pixel, all three sub-pixels arepermanently off, producing a permanently black pixel. Dead pixels canresult from similar manufacturing anomalies as stuck pixels, but mayalso occur from a non-functioning transistor resulting in complete lackof power to the pixel. Dead pixels are much less likely to correctthemselves over time or be repaired through any of several popularmethods.

Stuck pixels, unlike dead pixels, have been reported to disappear, andthere are several popular methods purported to fix them, such as gentlyrubbing the screen (in an attempt to reset the pixel), cycling the colorvalue of the stuck pixel rapidly (in other words, flashing bright colorson the screen), or simply tolerating the stuck pixel until it disappears(which can take anywhere from a day to years). While these methods canwork on some stuck pixels others cannot be fixed by the above methods.Also some stuck pixels will reappear after being fixed if the screen isleft off for several hours.

The above information applies to defective pixels in LCD monitors. Inthe case of defective pixels in camera sensors, of interest in thecontext of the present invention, the term dead pixels is generallyapplied to encompass all types of defective pixels. Furthermore, andstill in the context of camera sensors, true “stuck pixels” that meetthe ISO 9002 13406-2 standard definition only exist for 3-CCD cameras,which are much less widespread than single sensor cameras. In the caseof single sensor cameras, color is most often obtained via a Bayerfilter and there are thus no sub pixels to speak of.

Keeping to the case of camera sensors, true “dead pixels” and “hotpixels” as defined in the ISO 9002 13406-2 standard (i.e. pixels thatare always off and always on) appear to be much less prevalent than“partially on” (i.e. pixels that are neither fully on nor fully off).Note that because of the Bayer filter effect, these “partially on”pixels have a distinct color in single sensor color cameras. It is notclear if the intensity of these “partially on” pixels can sometimeschange over time. What is clear is that over reasonably short periods oftime (i.e. hours) their intensity—for a given scene intensity—remainsunchanged. However, if the scene intensity changes significantly, theintensity of these “partially on” pixels can change. Our hypothesis toexplain this is a form of “spillover” from neighboring pixels.

To summarize all of the above: in the case of camera sensors, we use theterm dead pixels to describe all defective pixels, and the most commontype of defective pixels are those that remain partially on.

Defective Pixel Detection

Different approaches to detect defective pixels in an image arepossible. The one that has produced fast, accurate and reliable resultsfor the Applicant, and that has been selected involves analyzing animage that should be completely black.

This approach requires asking the user to supply an image whereabsolutely no light reaches the camera's sensor. The system then looksat all of the pixels in a user-specified Region Of Interest (ROI) andlocates all of the pixels that have at least one channel (i.e. R, G, B)value above a user-specified threshold. Note that the ROI can of coursealso be set to correspond to the entire image.

In the case of a 24 bit per pixel image (i.e. 8 bits each for the R, Gand B channels) the possible threshold values range from 0 to 255.

Depending on the specifics of the camera, the threshold value can varyquite a bit. Also, it can depend on some of the camera settings (e.g.gain, exposure, and others, depending on the specific settings availablefor the camera). Care should thus be used when selecting the thresholdvalue. There is generally a sharp “break” in the relationship betweenthe number of affected pixels and the threshold value (i.e. the numberof affected pixels increases tremendously—possibly going all the way to100% of the pixels—if you select a threshold value below this inflectionpoint). The optimal value will thus be the one just above thisinflection point. Generally, a user's eye will easily and intuitivelytell the user if defective pixel correction is required and after a fewquick “trial and errors”, a good 1^(st) approximation of what thethreshold value should be.

Defective Pixel Correction Scheme

The approach described above can be used to detect defective pixels. Aflag (indicating whether the pixel is “defective” or not) can then beset and kept in an ordered vector, for every pixel.

Note that this vector can be saved to disk and carried over from sessionto session. Proceeding in this way, it is only necessary in theory tocarry out defective pixel correction one, on system commissioning.

With this vector of “flags”, defective pixel correction can be carriedout on every image coming from the camera. To do this, a defective pixelcorrection procedure should be introduced far upstream in the “imagecapture” pipeline. In fact, this procedure should immediately follow theimage capture operation proper (it should certainly be before anydistortion correction or similar procedure).

The defective pixel correction procedure should consider every pixel inthe ROI and for every pixel (i,j) with a flag indicating that the pixelif “defective”, the following localized (i.e. pixel-specific) correctionshould be carried out:

R(i,j)=(R(i−1,j)+R(i+1,j)+R(i,j−1)+R(i,j+1))/4

G(i,j)=(G(i−1,j)+G(i+1,j)+G(i,j−1)+G(i,j+1))/4

R(i,j)=(B(i−1,j)+B(i+1,j)+B(i,j−1)+B(i,j+1))/4

It should be understood that the above 4-neighbor correction schemeshould be modified appropriately if one or two neighboring pixels arenot available (such as would be the case for the pixels on the edge ofthe image, for example). Also, neighboring pixels that themselves needto be corrected should not be used to carry out defective pixelcorrection and the above equation should be modified accordingly.

Note that it is also possible to consider a 9-neighbor correction scheme(or even wide, for that matter). In this case, the use of appropriate(i.e. distance-weighted) interpolation coefficients would berecommended.

Although the present invention has been explained hereinabove by way ofa preferred embodiment thereof, it should be pointed out that anymodifications to this preferred embodiment within the scope of theappended claims is not deemed to alter or change the nature and scope ofthe present invention.

1. A digital optical comparator comprising: a holder for holding a part;a light source for illuminating the part; a camera for capturing animage of the illuminated part and a lens placed in front of said camera;and a data processing system for receiving the image, for obtaining aCAD drawing of said part, for displaying said image and said drawing ona computer screen, where said drawing is digitally overlaid on saidimage, said data processing system further including means for aligningsaid drawing with said image.
 2. A digital optical comparator accordingto claim 1, wherein said comparator further includes an imageenhancement module.
 3. A digital optical comparator according to claim1, wherein said means for aligning said drawing with said image includea joystick operatively associated with said data processing system.
 4. Adigital optical comparator according to claim 1, wherein said means foraligning said drawing with said image include a mouse and a keyboard ora trackball.
 5. A digital optical comparator according to claim 1,wherein said means for aligning said drawing with said image include apattern matching subsystem, said subsystem being adapted to determine aposition and orientation of said part, and to align said drawing withsaid part based on said position and orientation.
 6. A digital opticalcomparator according to claim 1, wherein said computer screen includes aplurality of screens, so that said image is displayed in 1:1 resolution.7. A digital optical comparator according to claim 1, wherein said partis mounted on a moveable carriage.
 8. A digital optical comparatoraccording to claim 1, wherein said data processing system is a generalpurpose computer.
 9. A digital optical comparator according to claim 1,wherein said comparator further includes a defective pixel corrector.10. A digital optical comparator according to claim 1, wherein saiddrawing displays said part, and also graphically displays tolerances forsaid part, so that a determination whether the part is manufacturedwithin tolerances can be visually made without measuring said part. 11.A digital optical comparator according to claim 1, wherein said dataprocessing system further includes a measurement package for enablingon-screen measurements.
 12. A digital optical comparator according toclaim 1, wherein said camera provides a streaming video signal to saiddata processing apparatus.
 13. A method for permitting comparisonbetween a part under study and a reference drawing, said part havingbeen manufactured to specifications contained in said drawing, saidmethod comprising the steps of: (a) placing said part on a support; (b)illuminating said part; (c) capturing an image of said part with acamera, said camera being provided with a lens; and (d) overlaying saiddrawing and said image of said part on a display.
 14. A method accordingto claim 13, wherein said method further includes the step of correctingsaid image with an image correction module prior to displaying saidimage on said display.
 15. A method according to claim 13, wherein saidmethod further includes the step of enhancing said image with an imageenhancement module prior to displaying said image on said display.
 16. Amethod according to claim 13, wherein said method further includes thestep of manually or automatically aligning said part with said drawing.17. A method according to claim 13, wherein said method further includesthe step of correcting a distortion introduced by said lens with adistortion correction module.
 18. A method according to claim 13,further comprising the step of providing a measurement module in orderto enable on-screen measurements of said part.